generalize toggleHover mode bar button handler,
generalize toggleHover mode bar button handler,

 so that cartesian plots keep retrieve their 'x' and 'y' hovermode
 when toggling.

iongroup/plotly.js,aburato/plotly.js,iongroup/plotly.js,etpinard/plotly.js,iongroup/plotly.js,plotly/plotly.js,etpinard/plotly.js,asolagmbh/plotly.js,aburato/plotly.js,plotly/plotly.js,asolagmbh/plotly.js,plotly/plotly.js,asolagmbh/plotly.js,etpinard/plotly.js,plotly/plotly.js